Astro-2: A Shuttle Mission Made in Heaven

by
William P. Blair

Johns Hopkins University

March 2, 2010, marked the 15th anniversary of the launch of
the space shuttle Endeavour on a dedicated Spacelab astronomy
mission known as Astro-2 (STS-67). The primary payload was a trio
of aligned ultraviolet telescopes attached to a pointing system that
turned the shuttle into a high-flying astronomical observatory. The
16-1/2 day mission, lengthened a day by bad weather at the landing
site, was the longest shuttle mission to date, and held that mark until
July 1996 when a medical research flight stayed in orbit just seven hours
longer! The long mission duration allowed the telescopes to observe
over 250 astronomical sources in the near and far ultraviolet part of
the electromagnetic spectrum, wavelengths that don't get through the
earth's atmosphere.

The telescope package was operated from the shuttle around the
clock by two teams of astronauts who worked back-to-back 12 hour
shifts. Primary control was by the crew directly from the Aft flight
deck of the shuttle, with a large contingent of scientists and NASA
personnel supporting their activities from Spacelab Mission
Operations Control at NASA's Marshall Space Flight Center in
Huntsville, Alabama. Roughly twice per orbit the shuttle was
maneuvered to point the telescopes at a new object for observation.

(Click on either picture to see a larger version.)

Captions: (Left) The Astro-2 telescopes in the payload bay of the
shuttle Endeavour during the Astro-2 mission. (Right) Astronauts John
Grunsfeld and Sam Durrance operating the telescopes from the Aft
Flight deck of the shuttle.

Components of the Astro Observatory

The instruments making up the payload included an imager and two
spectrographs, each of which had some characteristic that made
them unique. The
Ultraviolet
Imaging Telescope (UIT)
was designed
and built at NASA's Goddard Space Flight Center in Greenbelt,
Maryland. This 0.4 meter telescope with a UV-sensitive image
intensifier carried 2-1000 frame cassettes of 70mm IIa-O
astronomical film and was used to image 40 arcminute circular
fields of view with about 2 arcsecond resolution. While this
resolution is far below that achievable with the Hubble Space
Telescope, each image covered an area over 250 times larger than
that of the Wide Field Planetary Camera-2. Hence, UIT was able to
survey the UV populations of entire galaxies or star clusters in
individual pointings. The films exposed on orbit were returned to
earth at the end of the mission, where they were developed and
carefully digitized for analysis and comparison with optical
imagery.

The
Wisconsin
Ultraviolet Photo-Polarimeter Experiment (WUPPE)
included a 0.5 meter telescope and a special ultraviolet spectrograph that
could measure the polarization of ultraviolet light across the 1400 -
3200 Angstrom range. WUPPE was developed by the Space
Astronomy Laboratory of the University of Wisconsin, Madison.
While some instruments on Hubble can nominally measure
polarization, WUPPE specializes in bright stars and extended nebulas
that are difficult or impossible targets for Hubble.

The third telescope in the package was the
Hopkins Ultraviolet
Telescope (HUT)
. As its name suggests, HUT was produced by the
Johns Hopkins University in Baltimore, Maryland, and the
University's Applied Physics Laboratory in Laurel, Maryland. This 0.9
meter telescope, the largest of the three, was designed to perform
moderate resolution spectroscopy in the 850 - 1850 Angstrom
region of the far-UV. The spectrographs on Hubble cut off sharply at
wavelengths just below 1200 Angstroms. HUT overlaps with Hubble, but
accesses shorter wavelengths, a spectral region that is chock full of
important astronomical information.

History and the Astro-1 Mission

The telescopes that comprise the Astro Observatory were selected
by NASA for development in 1978, three years before the
space shuttle even flew! Prototypes of each telescope had flown on
sub-orbital "sounding rockets", but such flights only returned about
five minutes of data from roughly six months of effort and
preparation. The idea of the "Astro Observatory" was to develop a
relatively inexpensive package of telescopes that could fly multiple
times on the shuttle for a week or two each time. The Astro
telescopes were scheduled to fly in March 1986 to observe comet
Halley on its way out of the inner solar system, but the Challenger
accident in late-January 1986 halted all shuttle flights for 2-1/2
years.

After a series of frustrating delays, the payload finally flew aboard
the eight-day Astro-1 mission (STS-35) in December 1990. In
addition to the three UV telescopes, an X-ray telescope from NASA-
Goddard was carried along on a separate pointing system. (This
telescope was not reflown on Astro-2.) The operational phase of
Astro-1 was beset by a number of technical problems that were
widely reported. In particular, the pointing system had
difficulty locking onto guide stars properly, which affected not only
the quantity but the quality of the data gathered. However, the
"untold story" is that most of these problems were overcome, and
Astro-1 was a very successful mission.
(Click here
for a brief summary of HUT Achievements from Astro-1.) Over 120
scientific papers were published based on Astro-1 observations!

Like the Phoenix...

At the time of the Astro-1 flight, pressures on the shuttle launch
schedule were such that no additional flights of the telescopes were
planned. However, as some payloads were switched to expendable
launchers and as results from Astro-1 started coming out, NASA
reconsidered and Astro-2 was born!

The launch was scheduled for early 1995, which provided ample time
to understand the problems with the pointing system and streamline
the observation planning procedures based on the Astro-1
experience. One of the telescopes (HUT) was substantially improved
with new optical coatings that enhanced its performance by more
than a factor of two. In addition, NASA selected 10 "Guest
Investigators" from the astronomical community to use the
telescopes during Astro-2 along with the scientists from the
instrument teams. Finally, NASA scheduled the mission on
Endeavour, which was outfitted with a special "energy kit" that
would allow the shuttle to remain on orbit for over two weeks.

As the launch date for Astro-2 approached, hopes were running high.
But for those of us involved in Astro-1, the specter of launch delays
and operational problems could not be entirely forgotten.

(Click on the picture to see a larger version.)

Caption: The spectacular night launch of STS-67 for the Astro-2
mission, at 1:38 a.m. EST on March 2, 1995.

The Astro-2 Mission--"Charmed" from the Start

Endeavour was scheduled to launch at 1:37 a.m. EST on March 2,
1995. In the couple of days prior to launch, the weather at Cape
Canaveral was overcast. The prediction for launch day was so bad
that there was serious discussion about whether to "tank up" or just
delay the mission by a couple of days and hope for better weather.
In the end, they decided to go for it as scheduled. It was the right
decision. In the hour prior to launch, the clouds dissipated
"dramatically" (to quote one eyewitness account), and the launch
occurred only a minute later than nominally scheduled! The eight
minute ride to orbit was picture perfect, and the orbit achieved was
exactly as planned. We were off to a good start!

The first 30 hours on orbit were devoted to "activation and checkout"
activities. When a major ground-based observatory comes on line,
this phase can take six months or more. We gave ourselves a little
over one day! Although some minor glitches occurred, this phase
went quite smoothly. A few "growing pains" with the pointing
system extended over the first several days, but they were worked
out as we were making observations, causing a relatively small
impact. Our carefully revamped procedures for replanning
observations was working effectively. Compared with our Astro-1
experience, this was heaven!

Another remarkable aspect of Astro-2 was that even "nature"
cooperated. For instance, three bright galactic novas had popped off
in the month and a half prior to launch. Hence, not only did we have
three novas to observe, but they were at different phases of
development at the time of our observations. Likewise, we intended
to make repeated observations of the active Seyfert galaxy NGC
4151 to search for variability that would provide clues to the
structure within light days of the suspected black hole in the center.
During our first observation of this galaxy we were surprised to find
that it was five times brighter than it had been during Astro-1!
Even observations of planets or other objects that had to be
observed at certain phases (or in conjunction with other spacecraft
such as Hubble,the Extreme Ultraviolet Explorer, or the ASCA
satellites) came off without a hitch. We were truly charmed.

For over two weeks the astronauts and ground controllers worked
around the clock scheduling observations, maneuvering the shuttle,
acquiring new targets, and observing with the telescopes. Some 385
shuttle maneuvers were made to point the telescopes at over 250
unique targets during Astro-2. And with more stable pointing, the
quality of the observations was outstanding as well. I remember as
late as 10 to 12 days into the mission having nagging thoughts,
wondering when the "bomb would drop on us" so to speak, but it
never happened. Astro-2 was a mission made in heaven.

The Legacy of Astro-2

One of the strengths of the Astro Observatory was its ability to
tackle an extremely wide range of scientific problems. Over 50
specific science programs had been proposed by the investigators,
observing everything from planets to quasars and representative
targets of just about everything in between!

With the telescopes operating specifically in the ultraviolet (which
gets blocked by earth's atmosphere), the Astro telescopes returned a
treasure trove of unique information about hot stars, cataclysmic
binary stars, nebulae and supernova remnants, and even the
Interstellar gas and dust in our own Galaxy and the Magellanic
Clouds. Beyond the Milky Way, the hot stellar populations in many
kinds of galaxies were probed, as well as the processes occurring in
and around so-called active galaxy nuclei, thought to be the sites of
massive black holes. Even the enigmatic quasars, which may be
more distant and energetic counterparts of active galaxies, were the
targets of observation.

It is impossible to describe the full breadth of results coming out of
Astro-2 in the context of this short description. However, one high
priority program, the measurement of the so-called intergalactic
medium will be mentioned because of its importance to cosmology.
(You can
click
here
to see a helpful diagram and get a slightly more detailed
description of this important result.)

The presence of a diffuse intergalactic medium (or IGM) is a basic
prediction of the Big Bang theory. Not all of the normal (or so-called
"baryonic") matter created in the Big Bang should have coalesced
into galaxies and stars; some should have been left in intergalactic
space. This IGM should give itself away by causing absorption at
high redshifts in the spectra of distant quasars. While discrete
absorption lines of hydrogen are seen in great numbers in quasar
spectra (the so-called "Lyman alpha forest"), these absorptions arise
from discrete clouds of material along the line of sight. No evidence
of a truly diffuse component has ever been seen from hydrogen.
Hydrogen is the most abundant element in the Universe, so the
reason it is not seen may be due to the fact that it is completely
ionized (and hence cannot absorb light).

Dr. Arthur Davidsen
from Johns Hopkins (the Principal Investigator of
the HUT instrument) and colleagues used HUT to obtain the far-UV
spectrum of a distant quasar known as HS 1700+64. The redshift of
this quasar is such that absorption from ionized helium (the next
most abundant element, and harder to ionize than hydrogen) could be
sought. This spectrum not only detected this diffuse IGM
unequivocally from its helium absorption, but permitted a
measurement of how much total material is present in this
component (with certain assumptions). The answer is that there
could easily be three to five times as much material in this
extremely dilute component of the universe as is present in ALL of
the galaxies and stars! Incidentally, the desire to make this
particular measurement was the primary reason Dr. Davidsen
proposed HUT to NASA in 1978! Largely because of the importance of this
science project, HUT is nowenshrined in the Smithsonian's National Air
and Space museum in Washington, DC. It is unfortunate that Dr. Davidsen
passed away in 2001, shortly before this transfer took place.

As of 15 years after launch, there are well over 100 technical
papers published in refereed astronomy journals by
the science teams. The data have been archived for general use
by astronomers, through the
Multimission
Archive at Space Telescope (MAST).
The marvelous data sets produced by this
NASA Spacelab mission will continue to bear much scientific fruit
for years to come.

William Blair is a Research Professor at Johns Hopkins
University and was Deputy Project Scientist for Mission Planning
and Operations on the HUT team for Astro-2. His involvement with
the Astro missions dates back to his arrival at Hopkins in 1984.
After Astro-2, Dr. Blair became Head of Mission Planning and then
Chief of Observatory Operations for the
Far Ultraviolet Spectroscopic
Explorer satellite project, also based at Hopkins. Dr.
Blair's main research interests are in supernova remnants and
cataclysmic variable stars.